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196
New proofs for NMAC and HMAC: Security without collisionresistance
, 2006
"... HMAC was proved in [3] to be a PRF assuming that (1) the underlying compression function is a PRF, and (2) the iterated hash function is weakly collisionresistant. However, recent attacks show that assumption (2) is false for MD5 and SHA1, removing the proofbased support for HMAC in these cases. ..."
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Cited by 86 (8 self)
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HMAC was proved in [3] to be a PRF assuming that (1) the underlying compression function is a PRF, and (2) the iterated hash function is weakly collisionresistant. However, recent attacks show that assumption (2) is false for MD5 and SHA1, removing the proofbased support for HMAC in these cases. This paper proves that HMAC is a PRF under the sole assumption that the compression function is a PRF. This recovers a proof based guarantee since no known attacks compromise the pseudorandomness of the compression function, and it also helps explain the resistancetoattack that HMAC has shown even when implemented with hash functions whose (weak) collision resistance is compromised. We also show that an even weakerthanPRF condition on the compression function, namely that it is a privacypreserving MAC, suffices to establish HMAC is a secure MAC as long as the hash function meets the very weak requirement of being computationally almost universal, where again the value lies in the fact that known
P.: Pioneer: verifying code integrity and enforcing untampered code execution on legacy systems
 In: Proceedings of the Twentieth ACM Symposium on Operating Systems Principles, SOSP 2005
, 2005
"... We propose a primitive, called Pioneer, as a rst step towards veriable code execution on untrusted legacy hosts. Pioneer does not require any hardware support such as secure coprocessors or CPUarchitecture extensions. We implement Pioneer on an Intel Pentium IV Xeon processor. Pioneer can be used ..."
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Cited by 82 (0 self)
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We propose a primitive, called Pioneer, as a rst step towards veriable code execution on untrusted legacy hosts. Pioneer does not require any hardware support such as secure coprocessors or CPUarchitecture extensions. We implement Pioneer on an Intel Pentium IV Xeon processor. Pioneer can be used as a basic building block to build security systems. We demonstrate this by building a kernel rootkit detector. 1
Shake well before use: Authentication based on accelerometer data
 In Pervasive
, 2007
"... Abstract. Small, mobile devices without user interfaces, such as Bluetooth headsets, often need to communicate securely over wireless networks. Active attacks can only be prevented by authenticating wireless communication, which is problematic when devices do not have any a priori information about ..."
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Cited by 58 (7 self)
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Abstract. Small, mobile devices without user interfaces, such as Bluetooth headsets, often need to communicate securely over wireless networks. Active attacks can only be prevented by authenticating wireless communication, which is problematic when devices do not have any a priori information about each other. We introduce a new method for devicetodevice authentication by shaking devices together. This paper describes two protocols for combining cryptographic authentication techniques with known methods of accelerometer data analysis to the effect of generating authenticated, secret keys. The protocols differ in their design, one being more conservative from a security point of view, while the other allows more dynamic interactions. Three experiments are used to optimize and validate our proposed authentication method. 1
Efficient collisionresistant hashing from worstcase assumptions on cyclic lattices
 In TCC
, 2006
"... Abstract The generalized knapsack function is defined as fa(x) = Pi ai * xi, where a = (a1,..., am)consists of m elements from some ring R, and x = (x1,..., xm) consists of m coefficients froma specified subset S ` R. Micciancio (FOCS 2002) proposed a specific choice of the ring R andsubset S for w ..."
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Cited by 48 (14 self)
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Abstract The generalized knapsack function is defined as fa(x) = Pi ai * xi, where a = (a1,..., am)consists of m elements from some ring R, and x = (x1,..., xm) consists of m coefficients froma specified subset S ` R. Micciancio (FOCS 2002) proposed a specific choice of the ring R andsubset S for which inverting this function (for random a, x) is at least as hard as solving certainworstcase problems on cyclic lattices. We show that for a different choice of S ae R, the generalized knapsack function is in factcollisionresistant, assuming it is infeasible to approximate the shortest vector in ndimensionalcyclic lattices up to factors ~ O(n). For slightly larger factors, we even get collisionresistancefor any m> = 2. This yields very efficient collisionresistant hash functions having key size andtime complexity almost linear in the security parameter n. We also show that altering S isnecessary, in the sense that Micciancio's original function is not collisionresistant (nor even universal oneway).Our results exploit an intimate connection between the linear algebra of ndimensional cycliclattices and the ring Z [ ff]/(ffn 1), and crucially depend on the factorization of ffn 1 intoirreducible cyclotomic polynomials. We also establish a new bound on the discrete Gaussian distribution over general lattices, employing techniques introduced by Micciancio and Regev(FOCS 2004) and also used by Micciancio in his study of compact knapsacks. 1 Introduction A function family {fa}a2A is said to be collisionresistant if given a uniformly chosen a 2 A, it is infeasible to find elements x1 6 = x2 so that fa(x1) = fa(x2). Collisionresistant hash functions are one of the most widelyemployed cryptographic primitives. Their applications include integrity checking, user and message authentication, commitment protocols, and more. Many of the applications of collisionresistant hashing tend to invoke the hash function only a small number of times. Thus, the efficiency of the function has a direct effect on the efficiency of the application that uses it. This is in contrast to primitives such as oneway functions, which typically must be invoked many times in their applications (at least when used in a blackbox way) [9].
Finding SHA1 Characteristics: General Results and Applications
"... So far, the complex characteristics needed for the recent collision attacks on members of the SHA family have been constructed manually by Wang et al. In this report, we describe a method to search for them automatically. It succeeds for many message differences and also for multiblock attacks. Th ..."
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Cited by 46 (3 self)
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So far, the complex characteristics needed for the recent collision attacks on members of the SHA family have been constructed manually by Wang et al. In this report, we describe a method to search for them automatically. It succeeds for many message differences and also for multiblock attacks. This answers open questions posed by many researchers in the field. As a proof of concept, we give a twoblock collision for 64step SHA1 based on a new characteristic. The highest number of steps for which a SHA1 collision was published so far was 58. We also give a unified view on the expected work factor of a collision search and the needed degrees of freedom for the search. Until now, no clear view on these parameters was possible, especially in the prominent case of the recent results on SHA1. As a result, our approach can exploit all available degrees of freedom.
Taper: Tiered approach for eliminating redundancy in replica synchronization
 In USENIX Conference on File and Storage Technologies
, 2005
"... We present TAPER, a scalable data replication protocol that synchronizes a large collection of data across multiple geographically distributed replica locations. TAPER can be applied to a broad range of systems, such as software distribution mirrors, content distribution networks, backup and recover ..."
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Cited by 35 (0 self)
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We present TAPER, a scalable data replication protocol that synchronizes a large collection of data across multiple geographically distributed replica locations. TAPER can be applied to a broad range of systems, such as software distribution mirrors, content distribution networks, backup and recovery, and federated file systems. TAPER is designed to be bandwidth efficient, scalable and contentbased, and it does not require prior knowledge of the replica state. To achieve these properties, TAPER provides: i) four pluggable redundancy elimination phases that balance the tradeoff between bandwidth savings and computation overheads, ii) a hierarchical hash tree based directory pruning phase that quickly matches identical data from the granularity of directory trees to individual files, iii) a contentbased similarity detection technique using Bloom filters to identify similar files, and iv) a combination of coarsegrained chunk matching with finergrained block matches to achieve bandwidth efficiency. Through extensive experiments on various datasets, we observe that in comparison with rsync, a widelyused directory synchronization tool, TAPER reduces bandwidth by 15 % to 71%, performs faster matching, and scales to a larger number of replicas. 1
Herding hash functions and the Nostradamus attack
 of Lecture Notes in Computer Science
, 2006
"... Abstract. In this paper, we develop a new attack on Damg˚ardMerkle hash functions, called the herding attack, in which an attacker who can find many collisions on the hash function by brute force can first provide the hash of a message, and later “herd ” any given starting part of a message to that ..."
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Cited by 30 (6 self)
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Abstract. In this paper, we develop a new attack on Damg˚ardMerkle hash functions, called the herding attack, in which an attacker who can find many collisions on the hash function by brute force can first provide the hash of a message, and later “herd ” any given starting part of a message to that hash value by the choice of an appropriate suffix. We focus on a property which hash functions should have–Chosen Target Forced Prefix (CTFP) preimage resistance–and show the distinction between Damg˚ardMerkle construction hashes and random oracles with respect to this property. We describe a number of ways that violation of this property can be used in arguably practical attacks on realworld applications of hash functions. An important lesson from these results is that hash functions susceptible to collisionfinding attacks, especially bruteforce collisionfinding attacks, cannot in general be used to prove knowledge of a secret value. 1
SuperSbox cryptanalysis: Improved attacks for AESlike permutations
 In FSE’10
, 2010
"... Abstract. In this paper, we improve the recent rebound and startfromthemiddle attacks on AESlike permutations. Our new cryptanalysis technique uses the fact that one can view two rounds of such permutations as a layer of big Sboxes preceded and followed by simple affine transformations. The big ..."
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Cited by 25 (6 self)
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Abstract. In this paper, we improve the recent rebound and startfromthemiddle attacks on AESlike permutations. Our new cryptanalysis technique uses the fact that one can view two rounds of such permutations as a layer of big Sboxes preceded and followed by simple affine transformations. The big Sboxes encountered in this alternative representation are named SuperSboxes. We apply this method to two secondround SHA3 candidates Grøstl and ECHO, and obtain improvements over the previous cryptanalysis results for these two schemes. Moreover, we improve the best distinguisher for the AES block cipher in the knownkey setting, reaching 8 rounds for the 128bit version. Key words: hash function, cryptanalysis, AES, Grøstl and ECHO. 1
Short chosenprefix collisions for MD5 and the creation of a rogue CA certificate
 CRYPTO 2009, SPRINGERVERLAG 2009, LNCS XXXX, YY–ZZ
, 2009
"... We present a refined chosenprefix collision construction for MD5 that allowed creation of a rogue Certification Authority (CA) certificate, based on a collision with a regular enduser website certificate provided by a commercial CA. Compared to the previous construction from Eurocrypt 2007, this ..."
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Cited by 25 (6 self)
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We present a refined chosenprefix collision construction for MD5 that allowed creation of a rogue Certification Authority (CA) certificate, based on a collision with a regular enduser website certificate provided by a commercial CA. Compared to the previous construction from Eurocrypt 2007, this paper describes a more flexible family of differential paths and a new variable birthdaying search space. Combined with a timememory tradeoff, these improvements lead to just three pairs of nearcollision blocks to generate the collision, enabling construction of RSA moduli that are sufficiently short to be accepted by current CAs. The entire construction is fast enough to allow for adequate prediction of certificate serial number and validity period: it can be made to require about 2 49 MD5 compression function calls. Finally, we improve the complexity of identicalprefix collisions for MD5 to about 2 16 MD5 compression function calls and use it to derive a practical singleblock chosenprefix collision construction of which an example is given.
Alpaca: extensible authorization for distributed services
 In 14th ACM Conference on Computer and Communications Security
, 2007
"... Traditional Public Key Infrastructures (PKI) have not lived up to their promise because there are too many ways to define PKIs, too many cryptographic primitives to build them with, and too many administrative domains with incompatible roots of trust. Alpaca is an authentication and authorization fr ..."
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Cited by 23 (3 self)
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Traditional Public Key Infrastructures (PKI) have not lived up to their promise because there are too many ways to define PKIs, too many cryptographic primitives to build them with, and too many administrative domains with incompatible roots of trust. Alpaca is an authentication and authorization framework that embraces PKI diversity by enabling one PKI to “plug in ” another PKI’s credentials and cryptographic algorithms, allowing users of the latter to authenticate themselves to services using the former using their existing, unmodified certificates. Alpaca builds on ProofCarrying Authorization (PCA) [8], expressing a credential as an explicit proof of a logical claim. Alpaca generalizes PCA to express not only delegation policies but also the cryptographic primitives, credential formats, and namespace structure needed to use foreign credentials directly. To achieve this goal, Alpaca introduces a method of creating and naming new principals which behave according to arbitrary rules, a modular approach to logical axioms, and a domainspecific language specialized for reasoning about authentication. We have implemented Alpaca as a Python module that assists applications in generating proofs (e.g., in a client requesting access to a resource), and in verifying those proofs via a compact 800line TCB (e.g., in a server providing that resource). We present examples demonstrating Alpaca’s extensibility in scenarios involving interorganization PKI interoperability and secure remote PKI upgrade.